1. Field of the Invention
The present invention relates to a polishing apparatus, and more particularly to a polishing apparatus for polishing a workpiece such as a semiconductor wafer to a flat and mirror polished surface.
2. Description of the Related Art
Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections and also narrower spaces between interconnections with connect active areas. One of the processes available for forming such interconnection is photolithography. Although the photolithographic process can form interconnections that are at most 0.5 .mu.m wide, it requires that surfaces on which pattern images are to be focused on by a stepper be as flat as possible because the depth of focus of the optical system is relatively small.
It is therefore necessary to make the surfaces of semiconductor wafers flat for photolithography. One customary way of flattening the surfaces of semiconductor wafers is to polish them with a polishing apparatus.
Such a polishing apparatus has a turntable and a top ring which rotate at respective individual speeds. An abrasive cloth is mounted on the upper surface of the turntable. A workpiece such as a semiconductor wafer to be polished is placed on the abrasive cloth and clamped between the top ring and the turntable. During operation, the top ring exerts a constant pressure on the turntable, and an abrasive solution is supplied from a nozzle over the abrasive cloth. The abrasive solution enters the gap between the abrasive cloth and the workpiece. The surface of the workpiece held against the abrasive cloth is therefore polished while the top ring and the turntable are rotating.
One known polishing apparatus of the above-described type is disclosed in, for example, Japanese Patent Publication No. 62-46308. The polishing apparatus has a turntable with an abrasive cloth mounted thereon and is supported by a vertical shaft below the turntable, and the shaft is driven by a driving mechanism. A top ring holding a wafer to be polished is positioned above the turntable, and the wafer is pressed against the abrasive cloth by the vertical moving mechanism of the top ring. The pressing force against the wafer is provided by a pneumatic cylinder of the vertical moving mechanism. Also, Japanese Laid-open Patent Publication No. 6-99348 discloses another structure of a polishing apparatus.
FIG. 4 shows an example of a structure of a conventional polishing apparatus including a turntable 1 having an abrasive cloth mounted thereon, a turntable driving mechanism (not shown) for rotating the turntable 1 about the turntable support axis, a top ring 2 which holds a wafer to be polished and is positioned above the polishing position of the turntable 1, a top ring rotating mechanism which rotates a top ring shaft 21 via a top ring support member 16, and a top ring pressing mechanism 14 for pressing the top ring 2 holding the wafer against the turntable 1.
The top ring 2 is provided with shaft 21 extending upwards from the center of the top ring 2, and this shaft 21 is driven to rotate by means of a belt 13 which is coupled to a motor 6 disposed on the support member 16 which holds the top ring 2 in place. The support member 16 is provided with a cylinder forming mechanism 14 for providing a vertical movement of the top ring 2 so as to press down the top ring 2 onto the turntable 1 and a counterweight 17. There is also a mechanism for swinging the top ring 2 away from the turntable 1 for loading/unloading of a wafer. The support member 16 holding the top ring 2 is made to swing by rotating drive shaft 4 by means of gears 8, 9 driven by a motor 7. Bearing 22 disposed on the stationary base 3 freely rotatably supports the shaft 4. Therefore, the top ring 2 cannot only rotate about the shaft 21, but can also swing about the swing shaft 4.
As shown in FIG. 4, the conventional polishing apparatus described above has the top ring swing mechanism (motor 7) positioned below the turntable, however the top ring rotation mechanism (motor 6) is positioned above the turntable, and the top ring rotation mechanism and the top ring pressing mechanism also are positioned well above the top ring. The result is that the overall height of the polishing apparatus becomes large, leading to problems such as a difficulty of installing the polishing apparatus into a cleanroom. Furthermore, dust particles generated by the motor 6 are prone to fall on the turntable 1 to cause wafer quality problems, and mist scattered from an abrasive solution on the abrasive cloth often causes malfunctioning of the motor 6.